This invention is generally directed to processes for preparing squaraine compounds; and more specifically the present invention is directed to processes that enable obtaining squaraine compounds of the same composition. In one embodiment of the present invention there are provided processes for the preparation of squaraine compounds wherein undesirable water is removed from the reaction by the addition of drying components such as trialkylorthoformates permitting processes that are readily scalable, and which in some instances enable products in high yields. Also, with the processes of the present invention short reaction times are possible, and complex vacuum distillation steps as utilized in the prior art are avoided. The squaraine compositions prepared in accordance with the process of the present invention are useful for incorporation into layered photoresponsive imaging devices, which are sensitive to light in the wavelength region of from about 400 to about 1,000 nanometers. Therefore, the resulting devices are responsive to visible light, and infrared illumination orginating from laser printing apparatuses wherein, for example, gallium arsenide diode lasers are selected. Therefore, the specific photoresponsive devices envisioned can, for example, contain situated between a photogenerating laye, and a hole transporting layer, or situated between a photogenerating layer and a supporting substrate, a photoconductive composition comprised of the squaraines prepared in accordance with the process of the present invention.
Numerous different xerographic photoconductive members with squaraines, and processes for the preparation thereof are known. Thus, for example, there is illustrated in U.S. Pat. No. 4,415,639, the disclosure of which is totally incorporated herein by reference, the use of known squaraine compositions, such as hydroxy squaraines, as a photoconductive layer in an infrared sensitive photoresponsive device. More specifically, there is described in this patent an improved photoresponsive device containing a substrate, a hole blocking layer, an optional adhesive interfacial layer, an inorganic photogenerating layer, a photoconductive composition capable of enhancing or reducing the intrinsic properties of the photogenerating layer, which photoconductive composition is selected from various squaraine compositions, including hydroxy squaraine compositions, and a hole transport layer.
Other U.S. Pat. Nos. disclosing photoconductive devices with squaraines are 4,471,041; 4,486,520; 4,508,803; 4,507,480; 4,624,904; 4,524,218; 4,524,220; 4,521,621; 4,523,035; 4,390,610; 4,353,971; 4,391,880; 4,524,219; 4,525,592; 4,559,286; 4,585,895; 4,585,884; 4,607,124; 4,606,986; 4,621,038; 4,628,018; 4,552,822; 3,617,270; 3,824,099; 4,150,987; 4,175,956; 4,353,971; 4,390,610; 4,391,888; 4,500,621; 4,123,270; and 4,481,270. Although processes for the preparation of squaraines are illustrated in the aforementioned patents, there are no teachings therein where water is removed by the utilization of drying components, rather these prior art processes employ, for example, the use of azeotropic agents and distillation steps; and wherein the yield of scale up product is substantially lower in some instances than the yields obtained with certain process embodiments of the present invention.
More specifically, processes for preparing squaraine compositions are well known and generally involve the reaction of squaric acid, an alkyl ester of squaric acid, a dialkyl ester of squaric acid, or an aryl substituted squaric acid derivative with an aromatic amine. Thus, for example, squaraine compounds can be prepared by the reaction at a temperature of from about 50.degree. C. to about 130.degree. C. of an aromatic amine and squaric acid in a molar ratio of from about 1.5:1 to 4:1 in the presence of a mixture of an aliphatic alcohol and an optional azeotropic cosolvent. About 200 milliliters of alcohol per 0.1 mole of squaric acid are used, while from about 40 milliliters to about 4,000 milliliters of azeotropic material are selected. Illustrative examples of amine reactants include N,N-dialkylanilines, while examples of aliphatic alcohols selected are primary alcohols, especially 1-butanol and 1-heptanol. Azeotropic materials include aliphatic and aromatic compositions inclusive of benzene and toluene. Also, it is known that the stoichiometry of the reaction of squaric acid with aromatic amines produces 2 mole equivalents of water, while the reaction of an alkyl squarate with aromatic amines produces one mole equivalent of water; and the reaction of 3-(4'-N,N-dimethylaminophenyl)-4-hydroxycyclobutene-1,2-dione with aromatic amines generates one mole equivalent of water.
Moreover, it is known that water present in the reaction mixture should be removed quickly and completely. In some situations, as little as 0.05 percent of water in the reaction mixture can lower the yield appreciably. Thus, there is disclosed in U.S. Pat. No. 4,500,621 a process for obtaining squaraine compounds by the reaction of squaric acid with N,N-dimethyl-m-toluidine in 1-pentanol solvent, without water removal, and wherein the yield decreased from 56 percent at 0.05 mole scale to 43 percent at 0.5 mole scale. Water removal methods employed in the prior art include: (1) a Dean Stark trap for azeotropic distillation; (2) distillation to a Soxhlet containing molecular sieves or other drying agent, and the return of dried solvent to the reaction mixture; (3) a gentle nitrogen purge of a refluxing reaction; and (4) combinations thereof incorporating fractionation and/or vacuum distillation.
Also, there is disclosed in U.S. Pat. No. 4,523,035 a process for obtaining squaraine compounds by the reaction of squaric acid with various aromatic amines in the presence of a 1-heptanol solvent at reduced pressure with water removal by a Dean Stark trap. When this process is applied to bis(4-(N-methyl-N-(p-chlorobenzyl)amino)phenyl)squaraine, the reaction yield decreases from 75 percent at a 2 liter to 52 percent at a 20 liter scale. While it is not intended to be limited by theory, extrapolation of these medium scale results indicates larger scale forecast yields of about 20 percent at a 1,000 liter scale of reaction. In addition, when preparing squaraine compounds wherein toluene and 1-butanol mixed solvent systems are selected, and there are utilized molecular sieves, there results a decrease of yield upon scale up of, for example, from about 56 percent at 200 milliliters scale to 40 percent at a 1 liter scale. Similarly, the selection of a toluene and 1-octanol mixed solvent with molecular sieves and a Soxhlet apparatus at atmospheric pressure provide squaraines with a yield decrease of from 42 percent at 200 milliliters scale to 18 percent at a 5 liter scale. In contrast, with certain embodiments of the present invention, reference for example working Examples IV and V, there results squaraines of high yields which are independent of scale, that is the yield is 80 percent at 200 milliliters scale and 82 percent at a 20 liter scale.
Additionally, there is disclosed in U.S. Pat. No. 4,524,219, the disclosure of which is totally incorporated herein by reference, a process for the preparation of squaraine compositions which comprises reacting an alkyl squarate with an aniline in the presence of an optional acid catalyst. Water formed during the reaction was removed by a Dean Stark trap or nitrogen gas flow. While it is not intended to be limited by theory, it is believed that substantially the same or similar intermediates, namely squaric acid, alkyl squarate, and dialkyl squarate, are involved in this process as in similar processes starting from squaric acid with the result that water removal is necessary to obtain good yields.
Furthermore, there is disclosed in U.S. Pat. No. 4,525,592, the disclosure of which is totally incorporated herein by reference, a squaraine process wherein there is reacted a dialkyl squarate and a N,N-dialkylaniline in the presence of an acid catalyst, at a temperature of from about 60.degree. C. to about 160.degree. C., and in the presence of water saturated aliphatic alcohol solvents, such as methanol, ethanol, propanol, butanol and the like. Furthermore, it is known that this process requires the initial presence of water in a water saturated alcohol solvent, although this and subsequent water formed during the reaction is removed by a nitrogen gas flow.
Moreover, there is also disclosed in U.S. Pat. No. 4,524,220, the disclosure of which is totally incorporated herein by reference, a process for the preparation of squaraine compositions which comprises the reaction of squaric acid with an aromatic aniline in the presence of an aliphatic amine, and wherein a Dean Stark trap was employed for water removal.
Also, there are illustrated in U.S. Pat. Nos. 4,559,286 and 4,607,124, the disclosures of which are totally incorporated herein by reference, processes for obtaining mixed squaraine compositions by the reaction of squaric acid with two aromatic anilines, specifically 3-fluoro-N,N-dimethylaniline and 3-methyl-N,N-dimethylaniline in the presence of 1-heptanol solvent, wherein water is removed, for example, by the combined influence of a partial vacuum reflux to a Dean Stark trap at 40 Torr, reference Table 1. More specifically, with the process of the U.S. Pat. No. '286 the yield decreases, for example, from 45 percent to 25 percent upon an 80 fold scale-up to 8 mole scale. Also, the composition of the squaraine product varies substantially as a consequence of the difference in reactivity of the two amines. In particular, the fluorine content of the squaraine composition decreases from 1.46 to 0.76 percent. In contrast, with the process of the present invention as illustrated in Example IX there results a 50 percent yield of product, which product composition was independent of scale.
Additionally, there is disclosed in U.S. Pat. No. 4,606,986 a process for obtaining mixed squaraine compositions by the reaction of squaric acid with N,N-dimethylaniline and 3-methyl-N,N-dimethylaniline wherein the yield decreases, for example, from 82 percent with a 500 milliliter reaction to 70 percent with a 12 liter reaction.
Furthermore, there is disclosed in U.S. Pat. No. 4,585,895, the disclosure of which is totally incorporated herein by reference, a process for the preparation of mixed squaraine compositions which comprises reacting a dialkyl squarate, a dialkylaniline and a dialkylhaloaniline in the presence of an aliphatic alcohol and an optional acid catalyst. In this process, there is selected water in a water saturated alcohol solvent, which water is subsequently removed.
Additionally there is disclosed in U.S. Pat. No. 4,624,904 a process for obtaining unsymmetrical squaraines by, for example, the reaction of 3-(4'-N,N-dimethylaminophenyl)-4-hydroxycyclobutene-1,2-dione with various ring substituted N,N-dimethylanilines in 1-heptanol solvent at 40 Torr with a Dean Stark trap.
Although the above processes for preparing squaraine compositions are suitable for their intended purposes, there continues to be a need for other processes wherein photoconductive squaraine compositions can be prepared. Additionally, and more specifically there remains a need for simple, economical processes for preparing squaraine compositions with stable properties, which squaraines can be incorporated into photoconductive devices. Moreover, there remains a need for processes that enable in some instances the preparation of squaraine products in high yields. In addition, there remains a need for processes that will enable the preparation of squaraine photogenerating pigments wherein complex distillation steps, and especially vacuum processes, are avoided for water removal. There is also a need for the preparation of squaraine compounds wherein the formed water subsequent to treatment can remain in the reaction mixture. Additionally, there is a need for commercial processes for the preparation of squaraine compounds in yields of greater than 80 percent. Also, there is a need for large scale commercial processes with short reaction times, with high throughput (grams of squaraine per liter of reactor), and wherein inexpensive alcohol solvents and cosolvents can be selected. Further, there is a need for processes wherein there is selected low mole ratios of amine to squaric acid reactants, which processes enable lower energy requirements as refluxing conditions are avoided. Moreover, there is a need for processes wherein symmetrical squaraines, mixed squaraines, unsymmetrical squaraines, and other similar squaraines as illustrated in the appropriate aforementioned patents are prepared, and wherein water is removed therefrom by the addition of a trialkylorthoformate. There is also a need for processes enabling the preparation of squaraine compounds wherein the rate of the chemical generation of water is substantially equivalent to the rate at which the water is chemically removed by the addition of a trialkylorthoformate. With respect to the aforementioned process, the water removing trialkylorthoformate does not react adversely with the starting reaction component such as a squaric acid or aromatic amine; can be readily separated from the resulting product; reacts preferentially with water in the presence of excess amounts of alcohol; is inexpensive and readily available; and does not adversely effect the xerographic properties of the squaraine product generated.